The English sentence is awkward. It's either "Do not speak in my stead!" or "Do not speak in my place!" or "Don't speak for me!"
"Do not speak instead of me" is OK (and is accepted). It would depend on the context.
"Do not speak in my stead" Where is this a common English phrase? Sounds rad.
England. Usually in formal speach.
"Don't speak for me" is the best accepted answer if you ask me.
"Don't speak on my behalf"